1. Field of the Invention
The invention relates to a process for producing shaped metal parts, in particular reduced-weight shaped parts comprising light metal, and to the shaped parts produced using this process and their use in light metal structures.
2. Background of the Invention
In view of increased ecological demands, but also with a view to use within high-technology applications, such as aircraft construction, automotive engineering or in parts with high static demands, it is highly important to reduce the weight of shaped metal parts. In this context, in particular light metals are materials which ensure an ever wider range of applications. A further possible way of reducing weight is to use foamed metallic materials. The foams used are distinguished by a lightweight structure, rigidity, compressive strength, improved mechanical and acoustic damping, inter alia. The production of components from foamed metallic materials is also known.
GB 892934 relates to the production of complex structures with a foamed metal core and a closed, nonporous surface.
DE 198 32 794 C1 describes a process for producing a hollow profiled section which is filled with metal foam. This process comprises the steps of pressing the hollow profiled section from a cladding material by means of an extrusion press which has an extrusion die comprising a female mould and a mandrel, supplying the metal foam comprising a foam material to the hollow profiled section through a feed duct which is formed in the mandrel.
DE 297 23 749 U1 discloses a wheel for a motor vehicle which comprises at least one metallic foamed core which is arranged in such a manner that it is exposed on the inner side of the wheel and has a cast wall on the outer side of the wheel. For casting of the wheel, the foamed core of aluminum foam is placed into a chill mould and positioned in such a way that, during casting, the outer cast skin is formed between the chill mould and the foam core.
DE 195 02 307 A1 describes a deformation element, in the housing of which a filling comprising an aluminum foam as energy absorber is provided. The housing may consist of metal or plastic. The filling body is simply an insert part without any material-to-material bonding to the housing.
However, the use of casting cores made from metal foam is of particular interest for the production of internally foamed metallic shaped parts.
For example, DE 195 01 508 C1 claims a component for the chassis of a motor vehicle and a process for producing a component of this type. For this purpose, a core made from aluminum foam is introduced into a pressure die-casting die, and this core remains in the die-cast aluminum component after the aluminum has been forced into the die (lost core principle). The aluminum foam used is formed from a mixture of aluminum powder and a blowing agent and is produced in a manner known per se in a multistage process (a process of this type is described, for example, in the article “Wirtschaftliche Fertigungstechniken für die Herstellung von Aluminiumschäumen” [Economic manufacturing techniques for the production of aluminum foams], Aluminium, 76th volume 2000, pp. 491 ff). According to DE 195 01 508 C1, the foamed aluminum bodies produced in this way, having a density of 0.6 to 0.7 g per cm3 and a closed porosity, are then placed into a die, with the core of foamed aluminum being supported or secured to the inner wall of the casting die at the locations which are subject to low loads, so that a uniform distance with a desired wall thickness is retained between the core and the die. Only by maintaining this distance between the core and the die is it possible to ensure that a closed, sufficiently stable wall is formed in the shaped part which is produced.
The process of fitting core supports in order to support cores in die cavities which is employed for this purpose has already long been part of standard practise in casting processes (cf. Giessereilexikon, 17th Edition 1997, Stephan Hasse, p.658 and pp. 640 ff.). Overall demands imposed on the cores which are to be used are not only that they must either be sufficiently pressure-stable for use in pressure die-casting processes or must be suitably temperature-resistant with respect to liquid or semiliquid metal for use in casting filling processes which proceed at a slow rate, so that their position in the die does not change and a part of the volume which they take up is not released again during the filling process, but also that they must satisfy the requirement for accurate supporting within the die cavity, which in some cases is highly complex. This can be recognised, for example, from the wide range of commercially produced core supports (cf. for example the delivery range of Phoebus Kemstützen GmbH & Co. KG, Dortmund) and also from the use of core-support adhesion units as auxillary means for fixing the core bodies in a die. However, in particular the use of core supports for the precise positioning of a core in a die leads to at some points very high pressures on the outer skin of the corresponding core bodies during the die-filling process. This is a problem, particularly in the case of reduced-weight foamed bodies, if foamed bodies of this type cannot be produced with precisely accurate dimensions and an outer skin of suitable stability, which is able to withstand the described temperature and pressure loads during the filling process, irrespective of whether or not core supports are used, is not formed at the same time.